The invention relates to a tunable magnetron comprising a rotatable tuning body situated in an evacuated chamber connected to the interaction space of the magnetron and having an active part projecting into the tuning cavities of the magnetron for varying the tuning by rotation of the tuning body. The instantaneous angular position of the tuning body determines the tuning frequency of the magnetron and thereby the transmission frequency. An electric motor drives the tuning body.
Such a magnetron is for example described in Swedish patent SE 191.373. The electric motor, which can be a common DC-motor or an AC-motor, is in this case situated outside the vacuum-tight envelope and coupled to the rotatable tuning body via a magnetic coupling, the two parts of which are situated on each side of a vacuum tight wall separating the evacuated chamber from the surroundings.
The most common use of such a magnetron is to let the tuning body rotate continuously for producing a continuous tuning variation with time, and to trigger the magnetron at moments which do not have any connection with the period of the tuning variation, whereby pulses of arbitrarily varying frequency are transmitted. This will improve the resistance against disturbances.
However, under certain circumstances it is desirable to be able to transmit pulses with accurately predetermined frequencies by means of such a magnetron. One example on this is MTI-radar, where movable targets are discriminated by phase comparison between transmitted and incoming signal. In this case usually a number of pulses, for example 7-10 pulses, are transmitted on a given frequency and phase measurements are made, whereafter a rapid jump is made to a new frequency and the phase measurements are repeated on this frequency. A desire then is that the magnetron frequency shall be adjusted to an exact value and that the jump to a new frequency shall occur rapidly. In other measurements a sequence of pulses are transmitted having from pulse to pulse varying frequency, the accuracy of the measurement being determined by the accuracy in the size of the frequency step. Also in this case the magnetron frequency must be adjusted accurately and rapidly.
Previously two fundamentally different solutions of the problem of transmitting fixed predetermined frequencies with such a tunable magnetron have been proposed. In a first case the tuning body rotates continuously at the same time as the instantaneous tuning is continually supervised, for example by means of a local oscillator which is locked to the magnetron and follows the tuning variations. The triggering moment is then controlled such that the desired transmission frequency is always obtained. This solution has the drawback that the accuracy of frequency, which can be reached, will be poor and that the exact time for the triggering cannot be determined in advance.
In another solution, which is for example described in SE patent application 8302434-9 to which co-pending U.S. patent application Ser. No. 601,517, filed Apr. 18, 1984, corresponds, the tuning body cooperates with a mechanical locking device which is activated when the tuning body is rotated in a direction opposite to the normal rotation direction, and then locks the body in an angular position which is determined by a locking shoulder. The tuning frequency then can be adjusted by varying the position of the locking shoulder, for example by means of a setting motor. This solution has the drawback that the construction is expensive and bulky and is slow in adjusting from one frequency to another. Furthermore it suffers from poor precision due to the fact that the low torque gradient of the magnetic coupling gives rise to regulation errors due to friction in the rotor journals.